A lot of times, you hear about cryptocoins with fixed supply and/or small inflation, that they are pyramids because the late entry has to pay for their coins to the early adopter.
The situation here is reversed, in the sense that we have a lot inflation and dilution for non-SP locked money. So the same argument can't be used with the reverse rationale.
Ponzi are somewhat different than pyramids, but there is something that is slightly unsettling regarding the 10% interest of steem dollars as it drains a closed-loop economy - in case external investors want to buy steem dollars for their 10% annual rate (which is rare in terms of performance)...